Measurement of Atmospheric Attenuation 

 at Millimeter Wavelengths 



By A. B. CRAWFORD and D. C. HOGG 



(Manuscript received September 20, 1955) 



A frequency -modulation radar technique especially suited to measure- 

 ment of atmospheric attenuation at millimeter wavelengths is described. 

 This two-way transmission method employs a single klystron, a single an- 

 tenna and a set of spaced corner reflectors whose relative reflecting properties 

 are known. Since the method does not depend on measurements of absolute 

 antenna gains a7id power levels, absorption data can he obtained more 

 readily and with greater accuracy than by the usual one-way transmission 

 methods. 



Application of the method is demonstrated by measurements in the 6 -mm 

 to 6-mm wave band. The residts have made it possible to assign an accurate 

 value for the line-breadth constant of oxygen at atmospheric pressure; the 

 constant appropriate to the measurements lies between 600 and 800 MCS 

 per atmosphere. 



INTRODUCTION 



It is well known that certain bands in the microwave region are at- 

 tenuated considerably due to absorption by water vapour and oxygen 

 in the atmosphere. A theory of absorption for both gases was given by 

 Van Vleck.^ Numerous measurements have been made on the gases when 

 confined to waveguides or cavities- and several when unconfined in the 

 free atmosphere.^ Nevertheless, there is some uncertainty regarding the 

 line-breadth constants which should be used in calculating water vapour 

 and oxygen absorption. In particular, at atmospheric pressure there is 

 doubt as to the amount of absorption on the skirts of the bands where 

 the absorption is small. The present work was undertaken to test a new 

 method of measurement and to improve the accuracy of experimental 

 data measured in the free atmosphere. 



The method of measurement is one of comparison of reflections from 



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